Regex Tester & Cheatsheet

Real-time pattern evaluation, match extraction, and regex syntax reference

Regular Expression Pattern

3matches detected

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Test String Payload

Matched Results (3)

#1 (idx 47)alex.johnson@enterprise-cloud.io

#2 (idx 110)ndl.long.nguyendai@gmail.com

#3 (idx 175)support@ndlong.site

Essential Regular Expression Syntax Cheatsheet

Character Classes

. Any character except newline
\d Any digit [0-9]
\w Word char [a-zA-Z0-9_]
\s Whitespace character
[a-z] Character range a to z
[^0-9] Negated set (non-digits)

Quantifiers

* 0 or more occurrences
+ 1 or more occurrences
? 0 or 1 occurrence (optional)
{3} Exactly 3 times
{2,5} Between 2 and 5 times
+? Lazy quantifier

Anchors & Groups

^ Start of string or line
$ End of string or line
\b Word boundary
(abc) Capturing group
(?:abc) Non-capturing group
a|b Alternation (a OR b)

Preventing ReDoS (Regular Expression Denial of Service)

Catastrophic backtracking occurs when nested quantifiers (such as `(a+)+$` or `([a-zA-Z]+)*`) encounter non-matching input strings. The regular expression engine enters exponential time complexity (`O(2^n)`), consuming 100% CPU on backend servers and causing entire Node.js, Python, or Go workers to freeze. Always audit regex patterns for ambiguous overlapping alternatives before production deployment.